Repetitive DNA is common in the human genome. It is prone to length changes, expansions and contractions. These changes lead to genome instability that can cause disease. Huntingtons Disease (HD), Fragile X syndrome, and Amyotrophic Lateral Sclerosis (ALS) are examples of diseases caused by a repeat expansion. In addition to disease-causing expansions, DNA repeats are hotspots for chromosome fragility and rearrangements. Cancer cells exhibit increased fragility as well and chromosome rearrangements, thus a better understanding of repair fidelity within repetitive DNA could lead to insights into cancer etiology. We have established multiple assays for studying repeat instability and fragility using a yeast system, which can be manipulated genetically. Using these assays, we have shown that repair via homologous recombination (HR) is a significant source of CAG repeat expansions. Yeast cells containing an expanded repeat tract and lacking the strand annealing protein Rad52, required for homologous recombination, have high frequencies of chromosome fragility, cell cycle arrest, and cell death, further implicating HR as an important process in repeat maintenance. Moreover, we have identified a limited set of histone modifications that control the fidelity of repair within an expanded CAG repeat. There are several types of HR within cells, which can occur with different temporal and spatial locations. The HR event which is causing repeat expansions, and the cellular processes that control repair fidelity, are currently unclear. To fill these gaps in knowledge, we propose to develop, in collaboration with Jim Haber, controllable systems to induce HR within a repetitive DNA tract, in order to determine which types of HR repair generate repeat expansions. In addition, we will investigate how timing and location within the nucleus influence repair pathway choice and fidelity. Lastly, we will investigate how histone modifications control repair fidelity during homologous recombination, and, in collaboration with Sergei Mirkin, screen for additional factors that influence this process. The overall goal is to determine the mechanisms the cell uses to control repair fidelity and prevent expansions within repetitive DNA.